Organic semiconductors are attractive for use in organic photovoltaic cells (OPVs) due to the potential for low-cost, high-throughput processing. While promising, limitations in the absorption efficiency must be addressed to improve OPV power conversion efficiency. In an OPV the absorption of light excites an electron from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) as schematically illustrated in FIG. 1. These are analogous to the conduction band and valence band in inorganic semiconductors. This electron remains bound to the positively charged hole in the HOMO level by Coulombic forces. The bound electron-hole pair is electrically neutral and is typically referred to as an exciton. In order to extract power from the OPV this exciton is separated into its constituent electron and hole, which can then be collected at the cathode and anode respectively.
Exciton dissociation in an OPV is realized using a heterojunction. A heterojunction is an interface between two organic materials with offset HOMO and LUMO levels. The material with the higher LUMO level is typically referred to as the electron donor, while the material with the lower LUMO level is typically referred to as the electron acceptor. The donor-acceptor (DA) interface will dissociate an exciton if the energy level offsets between the donor and acceptor are greater than the exciton binding energy, thus providing a path for electron relaxation through exciton dissociation. These materials are also the active optical absorbing materials in an OPV. As the excitons are created throughout the donor and acceptor layers they must diffuse to the DA interface in order to be dissociated into charge carriers. Typical exciton diffusion lengths for organic materials range from 3-40 nm.
One limitation in OPVs is that the exciton diffusion length (LD˜10 nm) of the active materials is much shorter than the'optical absorption length (LA˜100 nm), leading to necessarily thin absorbing layers. The optical absorption length is defined when the ratio of the transmitted light intensity over the incident light intensity is equal to exp(−1). The short LD of organic materials has lead to new design schemes for OPVs, such as using bulk heterojunctions (BHJ), planar-mixed layers and phosphorescent materials. Bulk heterojunctions and planar-mixed layers are device architectures that increase the amount of DA contact area by using interpenetrating networks, while maintaining continuous material pathways for charge collection. The interpenetrating network allows for an increased number of dissociation sites, reducing the distance an exciton must travel for dissociation. For most small molecule OPVs the interpenetrating network is achieved by thermally depositing two materials simultaneously, creating a mixture. In polymeric OPVs this morphology can be achieved by solution deposition of one blended layer that contains both the donor and acceptor materials.
Phosphorescent materials typically have a longer LD than their fluorescent counterparts due to their increased exciton lifetime. Exciton lifetimes in phosphorescent materials are typically longer than fluorescent materials due to their less favorable optical transitions back to the ground state. Advances that exploit BHJs have generated OPV efficiencies exceeding 4%.